The present invention relates to detectors for spectrophotometers and particularly to a detector for a spectrophotometer of the flow-through type utilizing variable wavelength radiation for liquid chromatography.
In the subject type of spectrophotometers for liquid chromatography a substance whose quantitative presence in a sample is to be determined is dissolved in a suitable carrier solvent and flowed through a detector cell, which has end windows through which ultraviolet or visible light radiation is directed. Radiation exiting from the cell falls on a photodetector whose output is recorded by suitable instrumentation which is calibrated to indicate the amount of radiation absorbed by the fluid flowing through the cell. Absorbance is customarily indicated by a graph continuously recorded on a strip chart by a pen recorder. The quantitative presence of a substance of interest is determined by measuring the area under the graph peaks which represent the amount of radiation of a particular wavelength that is absorbed, particular materials being identified by particular wavelengths characteristically absorbed by them.
The sensitivity of a spectrophotometer detector cell is a function particularly of the stability of the base line of the graph; the graph base line is established by the absorbance of the solvent used, and will change in relation to any change in the refractive index of the solvent, which in turn will be changed by a change in the temperature of the solvent in the cell. When the baseline changes the true peak area can not be measured accurately and the peaks themselves become less clearly defined and hence difficult to identify and measure with any reliable degree of accuracy.
A particular problem affecting sensitivity in conventional spectrophotometers is the inefficient utilization of the radiation applied. In instruments adapted to take measurements with radiation of various wavelengths, the beam characteristics of the source are different for different wavelengths (which may be provided by substituting different radiation sources or by selecting a particular wavelength with a monochromator or with a filter). Therefore, it has been the custom to dimension and mount the detection cell and photodetector in relation to the source so that a minimum diameter source beam of radiation will fill the cell entrance, which means a larger diameter beam will lose some radiation to vignetting at the entrance and, at the other end, the diametric area of exiting radiation impinging on the detector may increase beyond the area of the detector surface, and thus be wasted due to a change in the index of the solvent in the cell. Consequently, much of the radiation applied is lost and only a portion of the exiting beam is recorded. Consequently, the sensitivity of the instrument which is rather limited in the best circumstance when the index of refraction of the solvent remains constant (ie. when flow noise is at a minimum) is disproportionally reduced by any change in temperatures of the solvent, which alters its index of refraction and thus increases flow noise.